Inductors are a fundamental electromagnetic component necessary to a wide variety of devices, such as actuators, relays, motors, DC-to-DC converters and radio frequency (RF) circuits. Inductors having large inductances typically include wires wrapped around a bulk dielectric or ferromagnetic core, and are used in power converters and relays. Radio frequency inductors having small inductances typically are helical coils having an air or ferrite core, and are used in RF circuits and communications equipment.
Inductors for the microwave region can become too small to fabricate and suffer low efficiency and Q values. Conventional RF inductor techniques must often be abandoned. For instance, the ferrite core, or tunable coil slug, is unusable above VHF due to eddy current losses in the ferrite. Even printed spiral inductors have limited usefulness at microwave frequencies, as magnetic field circulation through silicon substrates results in eddy-current loss, and a higher than normal parasitic capacitance.
Therefore, there exists a need for a microwave inductor of practical size and construction, with high Q and efficiency, and having adjustable or tunable features. With radio communications moving to higher and higher frequencies, the need is becoming ever more acute. A typical RF communication device, such as a cellular telephone uses inductors with an inductance in the range of 5–12 nH (nanohenries).
For example, U.S. Pat. No. 6,005,467 to Abramov is directed to a trimmable inductor including a supporting substrate having spaced apart lead terminals, a coil defined by an electrically conductive member mounted on the substrate in a continuous path of multiple turns forming a winding about an axis extending between the lead terminals, and an electric conductive shorting member extending and electrically connected between at least two adjacent windings of the coil to enable selective inclusion and elimination of one of the windings. Cuts are made in the conductors or shorting member to trim the inductor.